This invention relates to a vehicle antenna window and amplifier especially suited for amplifying electromagentic signals in the two frequency bands ranging from 0.1 to 6 MHz and from 88 to 104 MHz. The first of these bands will be recognized as the long, medium and short wave band in which amplitude modulated (AM) signals are typically transmitted and the second as the ultra short wave band in which frequency modulated (FM) signals are ordinarily transmitted.
In the prior art, numerous circuit designs attempt to limit the amplification of received signals to only these two frequency bands so as to minimize interference and cross-modulation. Typical amplifier circuits include a separate channel for amplification of signals in each of these two bands comprising at least a transistor amplifier and input and output frequency filters for each channel.
In the present invention the need for separate AM-band and FM-band amplifier channels is avoided by providing a .pi. circuit comprising two capacitors and an inductance between the base of the antenna conductor and the input of the transistor amplifier. The parameters of the .pi. circuit are chosen to obtain the best signal-to-noise ratio at the transistor input in the FM-band.
By using the .pi. circuit in combination with an antenna window and a transistor amplifier, one can obtain, at lower cost, not only a good band-pass characteristic in the FM-band with good signal-to-noise ratio but also favorable band-pass characteristics in the AM-band. As a result, a single channel and therefore a single transistor may be used to amplify the signals in both these frequency ranges.
To ensure adequate signal amplification in the AM-band, the antenna conductor should have a parallel capacitance of approximately from 20 to 60 pF and preferably from 30 to 40 pF and a parallel impedance having a resistive component of approximately 250 K.OMEGA.. The transistor amplifier should have a very low input capacitance, below approximately 10 pF, and a high input impedance having a resistive component of approximately 300 K.OMEGA..
In the FM-band, the antenna impedance is on the order of 300 .OMEGA., considerably smaller than that of the amplifier. This antenna impedance is matched to the high input impedance of the amplifier by the .pi. circuit of the present invention.
Preferably, the antenna is T-shaped having a vertical conductor in the center of the windshield and a horizontal conductor parallel to and adjacent the upper edge of the window. The horizontal conductor is shaped as a loop. Preferably, there is a break in this loop at some point other than the center line of the window. This asymmetry improves the general reception characteristics of the antenna.
In a particular embodiment of the invention a dual gate field effect MOS transistor is used as the amplifier. Particularly good results are obtained when the G2 input of said MOS transistor is used as the control electrode and the G1 input is used to adjust the operating threshhold. As will be recognized, this configuration is the opposite of conventional usage. With such a field effect MOS transistor and a T-shaped antenna conductor having an output impedance of approximately 300 .OMEGA. in the FM-band, the preferred embodiment of the .pi. circuit has a first capacitance that is approximately 4 pF, an inductance that is 0.5 .mu.H and a second capacitance that is 1 pF. In a particularly advantageous embodiment, the first capacitance of the .pi. circuit is provided by the capacitive effect of the impedance of the antenna base and the capacitance of the circuit conductors while the second capacitance is formed by the capacitance of the circuit conductors in conjunction with the input capacitance of the transistor amplifier. As a result, it is not necessary to provide discrete components for these two capacitances.
As another feature of the invention, the input of the .pi. circuit is preferably connected to the base of the antenna by a coupling capacitor having a cpacitance on the order of 20 pF. A coupling capacitor of such low capacitance has been observed to minimize the sensitivity of the amplifier to interference.